1. Field of the Invention
The present invention relates to a novel acid generator which is suitably used, especially as an acid generator for a resist composition, a chemically amplified resist composition using the same, and a patterning process.
2. Description of the Related Art
In recent years, as LSI progresses towards high integration and a further acceleration in speed, a miniaturization of a pattern rule is required. Deep-ultraviolet lithography and vacuum ultraviolet lithography are thought to hold particular promise as the next generation microfabrication technology. In particular, photolithography using an ArF excimer laser as the light source is the technique essential to ultra-fine microfabrication capable of achieving a size of 0.13 μm or less.
ArF lithography has started being partially used from production of a 130-nanometer node device, and used as a main lithography technique from production of a 90-nanometer node device. At first, 157-nm lithography with a F2 laser has held promise for the next technique of 45-nm node lithography. However, the delayed development caused by various problems has been pointed, and ArF immersion lithography has been proposed suddenly. In the ArF immersion lithography, a liquid having a higher refractive index than air, such as water, ethylene glycol, or glycerin, is placed between a projection lens and a wafer to set the numerical aperture (NA) of the projection lens to 1.0 or more and achieve a high degree of resolution (see, Non-Patent Document 1).
To prevent a precise and expensive optical material from deteriorating, the ArF lithography requires a resist composition having a high sensitivity capable of getting sufficient resolution even by a small exposure dose. As a strategy for the resist composition, it is most common that a component having a high transparent at a wavelength of 193 nm is selected. For example, a polyacrylic acid and derivatives thereof, a norbornene-maleic acid anhydride alternating polymer, polynorbornene, a ring-opening metathesis polymer, and a hydrogenated ring-opening metathesis polymer have been proposed as a base resin. This proposal achieves some results in terms of increasing the transparency of a resin itself.
Various photoacid generators have been investigated. When a photoacid generator generating an alkane or an arene sulfonic acid, which is used for a chemically amplified resist composition using the conventional KrF excimer laser as the light source, is used as a component of the ArF chemically amplified resist composition, the acid strength is not sufficient to cut the acid labile group of a resin, a resolution cannot be achieved, or a sensitivity is low. Thus, the resist composition is not suitable for production of a device.
For this reason, as a photoacid generator in the ArF chemically amplified resist composition, photoacid generators generating a perfluoroalkanesulfonic acid having a high acid strength are typically used. The photoacid generators generating a perfluoroalkanesulfonic acid have been developed for a KrF resist composition. For example, Patent Documents 1 and 2 describe photoacid generators generating perfluorohexanesulfonic acid, perfluorooctanesulfonic acid, perfluoro-4-ethylcyclohexanesulfonic acid, and perfluorobutanesulfonic acid. Further, as a novel acid generator, acid generators generating a perfluoroalkyl ether sulfonic acid are suggested in Patent Documents 3 to 5.
On the other hand, perfluorooctanesulfonic acid or derivatives thereof are known as its acronym PFOS, and have problems of stability (non-degradability) caused by a C—F bond, bioconcentration due to hydrophobicity and lipophilicity, and accumulation. Environmental Protection Agency (EPA) has adopted Significant New Use Rule, listing 13 PFOS-related chemical substances and further 75 chemical substances although their use in the photoresist field is excluded. Further, it has been suggested that Significant New Use Rule is applied to 183 chemical substances such as a perfluoroalkanesulfonic acid or a derivative thereof.
To cope with the problems of PFOS, various companies have developed a partially fluorine-substituted alkanesulfonic acid having a lower fluorine substitution ratio. For example, Patent Document 6 discloses development of an α,α-difluoroalkanesulfonic acid salt from an α,α-difluoroalkene and a sulfur compound, and a resist composition containing a photoacid generator generating such a sulfonic acid upon exposure, specifically di(4-tert-butylphenyl)iodonium 1,1-difluoro-2-(1-naphthyl)ethanesulfonate. Patent Document 7 discloses development of an α,α,β,β-tetrafluoroalkanesulfonic acid salt from an α,α,β,β-tetrafluoro-α-iodoalkane and a sulfur compound, a photoacid generator generating such a sulfonic acid, and a resist composition. Patent Document 3 discloses a photoacid generator having a difluorosulfoacetic acid alkyl ester (e.g., 1-(alkoxycarbony)-1,1-difluoromethane sulfonate), or a difluorosulfoacetic acid amide (e.g., 1-carbamoyl-1,1-difluoromethane sulfonate), but does not describe a synthesis method thereof. Patent Document 8 discloses a photosensitive composition containing a compound generating a partially fluorinated-alkanesulfonic acid having a sulfonylamide structure derived from perfluoroalkylene disulfonyl difluoride.
The substances described in Patent Documents have a lower fluorine substitution ratio. However, since the substances have a hydrocarbon skeleton which is unlikely to be degraded as a basic skeleton, and do not have a substituent which is easily degraded like an ester group, the substances have low degradability. Further, the molecular design in which the size of alkanesulfonic acid is changed is limited. Moreover, the substances have problems such as expensive starting materials containing fluorine.
For example, as the circuit line width is reduced, the influence of contrast degradation by acid diffusion becomes more serious for a resist composition. This is because the pattern size approaches the diffusion length of acid. As a gap (mask error factor (MEF)) in the size on a wafer relative to a gap in the size of a mask is increased, mask fidelity is decreased and pattern rectangularity is degraded. Accordingly, in order to obtain more benefits from a reduction of the wavelength of light source and an increase of NA, it is necessary that dissolution contrast be increased or acid diffusion be suppressed as compared with the conventional material.
Patent Document 9 discloses alkanecarbonyloxy-1,1,3,3,3-pentafluoropropane sulfonate having 1 to 20 carbon atoms or arenecarbonyloxy-1,1,3,3,3-pentafluoropropane sulfonate having 1 to 20 carbon atoms, such as triphenylsulfonium 2-(adamantane-1-carbonyloxy)-1,1,3,3,3-pentafluoropropane sulfonate, which is not sufficient to control diffusivity.
Patent Documents 10, 11, and 12 disclose photoacid generators generating a partially fluorinated-alkanesulfonic acid having a polycyclic hydrocarbon group, which cannot achieve sufficient resist performance. Since each photoacid generator is a labile ester of difluoroacetic acid, the storage stability of a resist composition containing the photoacid generator may be particularly decreased.
A certain gap in the exposure dose may be generated in the actual production of a device. Therefore, an exposure margin (EL) is required to keep a pattern profile that is substantially the same even when some gap in the exposure dose is generated. While a further miniaturization of a pattern rule is required, excellent performances of sensitivity, substrate adhesion, and etching resistance are required. In addition, a good exposure margin and mask fidelity are necessary without degradation of a resolution.
In immersion lithography, there are problems. Minute water droplets are left on a resist wafer after the exposure, causing damages to the resist pattern profile, collapse or deform into a T-top profile to the developed resist pattern. Therefore, a patterning process which can form a good resist pattern after development in immersion lithography is required.